Corona treatment is a method using an electrical corona discharge to modify a plastic surface to improve its ability to accept inks and adhesives. In corona treatment a high voltage electrode is mounted parallel to and spaced from a grounded electrode. The air gap between the electrodes is energized, forming a corona, which, when plastic material is passed through modifies the plastic material and makes it more receptive to ink and adhesives.
In a corona treatment system, a dielectric material is usually applied to at least one of the electrodes to create a high voltage capacitor which helps produce the electrical corona discharge.
In early corona treating systems, the dielectric, usually a piece of the material to be treated, was wrapped around the grounded roller electrode. Due to the porosity of the films and the inability to eliminate air between the wrapped layers, the dielectric frequently pinholed. Although this process enabled quick repair, the amount of downtime due to pinholes was usually too often.
In an effort to improve dielectric life, roller covering suppliers began offering dielectric materials which they would apply and bond directly to the grounded electrode roller. These dielectrics would include such materials as hypalon, silicone, EPDM, epoxy, unsaturated polyesters, glass and ceramic, all of which would be applied directly to the metal ground roller core. Of course, each of these materials offered different physical and electrical properties, which provided the user with advantages and disadvantages, which were unique to the specific material. These advantages would include such things as low cost, longer life, high dielectric strength, better resistance to knife cuts, etc.
A user, by employing one of these dielectrics, would be required to send the metal ground roll core to the roller covering supplier each time the dielectric needed to be replaced, so that the supplier would be able to remove the old covering and recover the core with the new material. In order to be able to do this the user would typically find it necessary to possess an inventory of metal roller cores, since the recovering process could take several weeks to do. Depending on the number of treater systems possessed, the standard delivery times provided by the roller covering supplier and the average life expectancy of the dielectric, the required metal core inventory necessary, could be quite large and costly to maintain.
Adding to the costs associated with the metal core inventory was the need to repair the metal core occasionally. During the process of removing the old covering from the metal core, the roller covering supplier removes a thin layer of the base metal as well. When this occurs, the physical dimensions of the metal core are then changed, which may have an effect on performance. By changing the shape of the core, the dielectric covering wall thickness may vary, which could result in variations in treat levels, premature dielectric failure, etc. In many cases, because the roll is an idlingroll (not driven) the metal core is made of a thin metal material (usually aluminum). Removal of metal from the core surface will eventually weaken the core making it unusable, so that replacements are often required.
Another problem faced by the user is, depending on where the treater is located, the accessibility of the roll. In many cases, the treater station is positioned in locations that make roll changes difficult and time consuming, causing extended periods of downtime and lost production. In an effort to overcome this, several companies offered disposable dielectric coverings, that were designed to slide on top of a metal core. The most common sleeve coverings were rubber, such as silicone and hypalon. However, several companies did offer a fiberglass sleeve which was designed to be used on top of the metal core.
Both the rubber and the epoxy/fiberglass sleeves required a metal roller core which the dielectric would be mounted on. In the case of the rubber sleeves, the metal core diameter was designed to be just slightly larger than the sleeve, so that the sleeve would fit snugly on the mandrel (metal core), thus preventing any additional air gaps between the dielectric and the grounded metal core. Since the sleeves were snug, mounting them to the mandrel required the use of pressurized air to help float the sleeve on. Since the rubber sleeves do not exhibit great resistance to tearing and cuts, they frequently were damaged during the mounting process.
The epoxy/glass sleeves, because they were not elastic, were designed to be slightly larger in diameter than the metal core so that they could easily slide over the mandrel. Once on the mandrel, the epoxy/glass sleeve was pinned to the core. As a result, an air gap would exist between the metal mandrel and the epoxy/glass sleeve. This resulted in a corona being produced in this gap, taking away some of the power that was intended for use on the plastic film. In addition to reducing the efficiency of the system, the undesired corona provided additional stresses on the dielectric that would typically result in premature failures.
There is a need for an inexpensive, disposable roller electrode for use in corona discharge treatment systems.